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The disk surface density of the nearby spiral galaxy M33 is estimatedassuming that it is marginally stable against gravitational perturbations. Forthis purpose we used the radial profile of line-of-sight velocity dispersion ofthe disk planetary nebulae obtained by Ciardullo et al. (2004). The surfacedensity profile we obtained is characterized by the radial scalelength which isclose to the photometrical one and is in a good agreement with the rotationcurve of M33 and with the mass-to-light ratio corresponding to the observedcolor indices. However at the galactocentric distance $r>7$ kpc the dynamicaloverheating of the disk remains quite possible. A thickness of the stellar diskof M33 should increase outwards. The dark halo mass exceeds the mass of thedisk at $r>$ 7 kpc. The obtained radial profile of the disk surface density andthe radial gradient of $O/H$ are used to calculate the effective oxygen yield$Y_{eff}$ in the frame of the instantaneous recycling approximation. It isshown that $Y_{eff}$ increases with radius which may indicate that the role ofaccretion of metal-poor gas in the chemical evolution of interstellar mediumdecreases outwards.

On the mass and the density of stellar disk of M33